1. Field of the Invention
The invention relates to a novel power converter apparatus. More specifically, the invention relates to such apparatus having two or more converter arrangements, each converter arrangement being formed in a double-wye configuration.
2. Description of Prior Art
In conventional converter apparatus, when output current exceeds a certain level, it is no longer feasible to use only a single rectifier device (diode, thyristor, etc.) in each current path. Instead, several rectifier devices must be paralleled in order to avoid reaching the melt down current of the rectifiers in each current path.
However, such paralleling of rectifier devices is not a trivial task. The devices must be selected to have substantially identical characteristics. Resistors, inductors, or other components must be added to ensure current sharing. Furthermore, the physical lay-out and the arrangement of the rectifiers becomes critical.
Even if proper current distribution can be obtained at the time that the converter apparatus is first installed, the rectifier devices and other current elements will age differently so that after several years of operation, the current sharing will deteriorate and failures will occur. Since the rectifier devices must be matched, all such rectifier devices of the stack where the fault occurred must be replaced by a new set of, once again, carefully selected rectifier devices. Such failures typically occur much more frequently than predicted by the MTBF (mean time between failure) of the rectifiers. In fact, they are not caused by a problem in the rectifier devices, but rather, by circuit problems. Because of this, good current sharing cannot be ensured over a long period of time.
Examples of patents of interest having regard to the subject matter of the present application are Jensen U.S. Pat. No. 3,573,602, Apr. 6, 1971, Garnham et al U.S. Pat. No. 4,208,709, June 17, 1980 and Tanaka U.S. Pat. No. 4,663,702, May 5, 1987. The Jensen patent teaches a plurality of inverters connected in parallel. However, each leg consists of only a single primary. The Garnham et al patent describes, especially in FIG. 6, parallel connected converters. Once again, the transformer of each converter seems to consist of a simple transformer rather than a complex transformer. In addition, although the patent does teach a series connection of transformer primaries, the series connection is used to effectively increase the apparent switching frequency of a pulse width modulated converter as seen from an AC inductor which, as indicated in claim 1 of the patent, constitutes an essential part of any embodiment of the invention of this patent. The present invention makes no use of an AC side reactor nor does it in any way relate to pulse width modulated converters. Rather, the present invention is aimed at powering large direct current loads by allowing a multitude of simple phase controlled converters or uncontrolled rectifiers to be directly paralleled without any need for interphase transformers or AC side inductors. The circuit complexity which would normally be required to guarantee efficient load sharing among the converters is totally eliminated by the proposed primary interconnections. This results in much increased reliability as well as increased efficiency. Further, the proposed multi-phase connection of the present invention naturally results in increased ripple frequency and reduced ripple amplitude making it possible to substantially eliminate any filtering of the input currents or at least allow a simple capacitive filter to reduce current harmonics below acceptable levels.